Diaphragm carburettors are known which comprise a carburettor body with venturi duct to which a diaphragm pump and a diaphragm meter are fixed.
The meter is subject to the reduced pressure present in the insertion venturi duct of the mixture to the engine on the side of the diaphragm soaked by the fuel and to a pre-established pressure, for example atmospheric pressure, on the opposite side. This pressure difference on the two opposite sides of the meter diaphragm permits the diaphragm to oscillate, in such a manner driving a needle valve which shuts the fuel passage opening. In particular, the needle valve is held in closed position by a spring, blocking the passage of the fuel, and is driven open by the diaphragm against the spring. In practice, when the side of the diaphragm communicating with the venturi duct is in reduced pressure, the needle valve is driven open and the fuel reaches the distribution chamber placed on the same side communicating with the venturi duct, which draws the fuel through suitable fuel insertion opens.
The degree of opening of the needle valve is proportional to the reduced pressure present in the distribution chamber, which varies as a function of the adjustment of a butterfly valve which intercepts the insertion duct of the carburettor crossed by air.
The diaphragm pump sends the fuel to the needle valve, waiting for it to be opened by the diaphragm meter. In particular, the diaphragm pump has one side in communication with the fuel tank and the opposite side in communication with the engine crankcase which sends pressure pulses generated by the piston moving in the engine through a suitable duct crossed by the mixture. These pulses make the pump diaphragm oscillate, and the pump pushes the fuel towards the needle valve.
In the prior art, the pump and the meter are normally fixed on two opposite sides of the block comprising the insertion venturi tube of the mixture.
This configuration of the prior art presents several drawbacks.
In fact, the known pump and meter do not lend themselves to being applied both on carburettors provided with only the venturi duct of standard type, and on carburettors with two insertion ducts, one venturi for the air/fuel mixture, the other only for the air, so to obtain a stratified charge feeding.
In the latter case, both ducts are shut by a butterfly, and the two butterflies must move synchronously.
This is due to the fact that the distance between the pump and the meter increases since the space interposed between the two, which before was occupied by a single duct carburettor body, is now occupied by a body comprising two ducts, with a substantial doubling of distance; this is also reflected in the prearranged adjustment and lever system for the correct functioning of the butterfly valve, which will inevitably be modified.
In substance, the passage from the standard feeding to the stratified charge feeding involves, in the prior art, the modification of all carburettor components, i.e. the carburettor body, as well as the pump and meter.
In fact, due to the greater size of the stratified charge carburettor, which has two ducts instead of one, the arrangement of the pump and meter, opposite each other and with the two-duct carburettor interposed as occurs in some cases, make the hydraulic connections between the pump and meter difficult, as well as the adjustment of these connections.
In addition, in the prior art, the stratified charge carburettor has a considerably complicated timed management of the butterfly valve.
A modular diaphragm carburettor is disclosed by U.S. Pat. No. 6,446,939 in which the pumping means and the metering means of the fuel are made in two blocks both positioned on the same side of the venture passage.
Nevertheless they comprise a plurality of mating plates in which the passages through the carburettor is made, and the machining of such passages is far to be easy.
There is therefore the strong need to have available a single block comprising a diaphragm pump and a diaphragm meter, which are adapted to be indiscriminately applied both on a standard application, typical of the carburettor bodies with only one venturi duct, and on a stratified charge application, with two flanking parallel ducts, in the scope of a simple, extremely compact and versatile structural solution.